Metal halide lamps using ceramic arc tubes have been used widely for indoor lighting in stores and shops because such metal halide lamps have higher luminous efficiency, higher color rendering and longer service lives when compared to metal halide lamps using quartz arc tubes.
FIGS. 5 and 6 show respectively a metal halide lamp using a conventional ceramic arc tube. An arc tube 28 comprises an arc tube container 29 composed of a discharge arc tube portion 30 of a polycrystalline alumina ceramic material and a pair of thin tube portions (31, 32) sintered at the both ends of the discharge arc tube portion 30. A pair of tungsten coil electrodes (33, 34) are arranged at the both ends of the arc tube 28. Feeding portions (35, 36) of niobium or conductive cermet are adhered hermetically to the thin tube portions (31, 32) by means of frit 37, and the tungsten electrodes (33, 34) are connected to the respective feeding portions (35, 36). A luminescent material 38 comprising a metal halide, mercury as a buffer gas, and a start-aiding rare gas such as argon are filled in the arc tube 28. As illustrated in FIG. 6, the arc tube 28 composing a lamp 39 is disposed inside an outer bulb 40 of either quartz or hard glass, and a base 41 is attached to the outer bulb 40. About 50 kPa of a nitrogen-based gas is filled in the outer bulb 40. In general, the lamp 39 is turned on by means of a copper-iron inductance ballast or an electron ballast with a built-in starter.
For example, references such as JP-57(1982)-92747 A and U.S. Pat. No. 5,973,453 describe the use of cerium iodide in combination with sodium iodide for a luminescent material applicable for a typical metal halide lamp for indoor/outdoor use. The luminescent material of cerium iodide can provide improved luminous efficiency since many of the emission spectra of cerium are distributed in a region with a higher relative luminosity factor regarding human eyes. U.S. Pat. No 5,973,453 and Tokuhyo-2000-501563 (published Japanese translation of PCT international publication for patent application) describe a suitable NaI/CeI3 molar composition ratio in a range from 3 to 25 (corresponding to a CeI3 composition ratio from 12.2 wt % to 53.7 wt %), which is suitable for obtaining white light source color.
However, a conventional metal halide lamp filled with a luminescent material of cerium iodide and sodium iodide has a problem of a drastic change in the lamp color temperature as well as a remarkable lowering in the flux maintenance factor over the lighting time.